With the progress of the information society, communications networks are playing the role of the nervous system of a wide variety of social and economical activities and great importance is placed especially on their reliability. Lately, there are increasing demands for a backbone LAN capable of accommodating medium- arid small-scale branch LANs and integrated processing of data, voice, and image, and some backbone LANs to meet such demands are being produced as multiple-media backbone LANs.
On the other hand, high speed data transfer between host computers has come to be demanded with the progress of horizontal distribution of computer processing. To meet such demand, a token ring LAN having a channel interface capable of being directly incorporated into a host computer is made available. Accordingly, as a means for achieving high-speed transfer within a multiple-media backbone LAN, it becomes necessary to have branch LANs such as token ring LANs incorporated in the multiple-media backbone LAN.
Conventionally, when a plurality of branch LANs such as token ring LANs have to be connected to a backbone LAN, the connection between these LANs has been achieved in general by means of a bridge function whereby a small loop is connected to a large loop through a node.
When communications are made between branch LANs connected as described above, one branch LAN sends a token ring frame to the backbone LAN. Then, the node of the backbone LAN reads the address data in the token ring frame and retrieves the address of the node to which the branch LAN located at the read address is connected, puts the address into the backbone LAN frame, and incorporates the token ring frame into the backbone LAN frame, arid thereupon, sends the backbone LAN frame to the destination node. The node at the destination separates the token ring frame from the backbone LAN frame and transmits the token ring frame to the terminal unit having its address concurring with the address in the token ring frame.
In the above described conventional communications system between LANs of different types, since the communication is performed such that the node reads the address data of the destination of the token ring frame, retrieves the node with which the terminal having that address is connected, and puts the address of that node into the backbone LAN frame, the node is heavily loaded, and this has been a cause impairing high-speed large-capacity communications between branch LANs. Hence, even if the transmission capacity of the backbone LAN is 100 Mbps, for example, throughput at 100 Mbps has not been attainable. Thus, there has been a problem that the capacity of the node is insufficient to attain high-speed large-capacity communications between host computers connected to each of the branch LANs.
Accordingly, an object of the present invention is to provide a communications system between LANs of different types capable of overcoming the above described problem in the prior art arid properly achieving high-speed large-capacity communications between branch LANs by way of a backbone LAN.